Touch screens and touch panels gain popularity because of their ease and versatility of operation. They are used as input devices for performing operations in electronic devices such as mobile communication terminals, media players, personal and public computers, etc. In general touch screens recognize the touch and position of the touch on the display screen and the electronic device interprets the touch and thereafter performs an action based on the touch event. Touch screens typically comprise a touch panel or touch pad that is a clear panel with a touch sensitive surface. The touch panel registers touch events and sends these signals to the controller. The controller processes these signals and provides the data to the electronic device. Typically the transparent touch screen is positioned in front of a display device of the electronic device, where the display device is configured to display a graphical user interface (GUI). The touch screen acts as an input device that is sensitive to user□s touch, allowing the user to interact with the GUI on the display.
Typically the touch panel or touch pad (small touch panel) comprises a number of touch buttons or touch keys that activate an associated function when the user touches a corresponding button or key. The touch panel has a sensing surface that acts as a touch-sensistive user interface. In capacitive technologies, the touch panel can be made of conductive or semiconductive materials or be coated with a material that stores electrical charge. Suitable materials are e.g. copper, indium tin oxide (ITO), antimony tin oxide (ATO), (semi)conductive polymers, etc. Depending on measurement arrangements, e.g. electrode charging can be used as a measurement principle. Some other measurement principles can also be applied, e.g. sending and receiving electrodes, where a finger is an interfering object between the electrodes.
For example, when the panel is touched, a small amount of charge is drawn to the point of contact and the charge is measured in electric circuits of the touch panel before sending the information the controller. In the activated state the sensing surface detects, for example, a change of capacitance when the user touches it by a finger or other object. This change is detected by a sensor, e.g. capacitive sensor, or sensors associated to each button and the sensor provides an output signal responsive to a degree of capacitive coupling between the touch of the finger or object and the button. The output signal relates typically to the detected signal strength associated with the button. The function associated to the button is activated if the detected output signal of the button is in excess of a predetermined signal strength threshold, i.e. that the button is interpreted to be touched.
Electronic devices comprising the touch panel or touch buttons are rather unshielded from accidental activations of the touch buttons, particularly if the touch buttons are in unlocked operation state. For example, when answering a mobile phone a finger or cheek of the user may accidentally touch or sweep the touch buttons and hence activate the touch buttons improper way to cause false inputs. This happens when the touch of the user causes the sensor of the touch button to measure a signal strength that reaches the preset signal strength threshold level and consequently triggers the button.
FIGS. 1a-1d show sensor electrode structures of different touch buttons that are used in the prior art solutions. FIG. 1 a depicts a single uniform sensor (11) that forms a touch surface 15 a of the touch button. Here, the touch is triggered when it cause the signal strength to exceed signal strength threshold level. This kind of touch button does not have any accidental activation prevention.
FIG. 1b shows a touch pad comprising the touch surface 15 b where each of the touch buttons comprises a single uniform sensor 12, 13, 14, 16. Here, if upon touching the touch pad two or more sensors 12, 13, 14, 16 are activated with enough strong signal strength level at the same time then the touch is discarded. Document U.S. Pat. No. 6,993,607 discloses a basic method for accidental activation prevention shown in FIG. 1b. If the user□s finger overlaps several touch buttons at the same time by repeatedly measuring a detected signal strength associated with each button and defining which of the several touch buttons have a maximum signal strength compared to others. If the touch button of the maximum signal strength is found then there is interpreted that only this button is pressed and the others of the several buttons are discarded. Therefore, the document teaches how to prevent accidental false inputs from buttons adjacent to a selected button. However, U.S. Pat. No. 6,993,607 does not teach how to prevent accidental activation of a single touch button.
FIGS. 1c-1d depict the prior art solutions in which it is possible to prevent accidental activation of the single touch button to some extent. The functionality of the single touch button can be divided into multiple sensor electrodes. FIG. 1 c shows a touch surface 15c of a ring shaped electrode that is divided into three sensor sectors 17a-17c. FIG. 1d shows a touch surface 15d of a single button in which a sensor associated to the functionality of the single button is composed of four adjacent separated sensor electrodes 18a-18d. When the user□s finger touches the touch button it is activated only if measured signal strength of each of the sensor electrodes 17a-17c, 18a-18d is simultaneously in excess of the threshold level preset to each of the sensor electrodes 17a-17c, 18a-18d. However, this does not guarantee that the user does not activate the touch button when firmly touching or sweeping the button accidentally. For example a call/hang-up button of the mobile phone can be implemented using prior art touch button structures. Then during a phone call the user□s cheek may cause an unwanted hangup of a call in the mobile phone if the cheek accidentally activates all the sensor electrodes of the call/hang-up button simultaneously. This may be bothersome for the user.